The present invention relates to an arrangement for a supercharged combustion engine and particularly to the supply of exhaust gases mixed with air to the engine.
The technique called EGR (Exhaust Gas Recirculation) is a known way of leading part of the exhaust gases from a combustion process in a combustion engine back, via a return line, to an inlet line for supply of air to the combustion engine. A mixture of air and exhaust gases is thus supplied via the inlet line to the engine's cylinders in which the combustion takes place. Adding exhaust gases to the air causes a lower combustion temperature which results inter alia in a reduced content of nitrogen oxides (NOx) in the exhaust gases. This technique is used for both Otto engines and diesel engines.
Optimum use of this technique entails recirculation of relatively large amounts of exhaust gases. The recirculating exhaust gases are therefore cooled in at least one EGR cooler to reduce the specific volume of the exhaust gases before they are mixed with air and led to the combustion engine. Conventional EGR coolers use the coolant of the vehicle's ordinary cooling system for cooling the combustion engine. Another known practice is to use air-cooled EGR coolers in which the exhaust gases are cooled by air which is at the temperature of the surroundings, thereby allowing the recirculating exhaust gases to be cooled to a temperature substantially corresponding to the temperature of the surroundings. The recirculating exhaust gases can thus be subjected to a substantially optimum reduction in specific volume so that a large amount of exhaust gases can be recirculated into the combustion engine.
The amount of air which can be supplied to a supercharged combustion engine depends on the pressure of the air but also on the temperature of the air. Supplying the largest possible amount of air to the combustion engine entails the air being first compressed by a compressor before being cooled in a charge air cooler and thereafter being led to the combustion engine. The compressed air is usually cooled in the charge air cooler by surrounding air. The compressed air can thus be cooled to a temperature which exceeds the temperature of the surroundings by only a few degrees. Despite the air being compressed and cooled as described above, this is not usually sufficient for providing the necessary amount of air which together with the recirculating exhaust gases will enable combustion with optimum use of the EGR technique. Another advantage of supplying as large an amount of air as possible to the combustion engine is that the performance of the combustion engine improves.
US2006/0185362 refers to an arrangement for a supercharged combustion engine. The arrangement comprises an inlet line intended to lead air at above atmospheric pressure to the combustion engine. A first compressor is adapted to compressing the air in the inlet line as a first stage and a second compressor is adapted to compressing the air in the inlet line as a second stage. The arrangement comprises a cooling system with a coolant-cooled charge air cooler adapted to cooling the air after it has been compressed in the first stage and before it is compressed in the second stage. An air-cooled charge air cooler is thereafter adapted to cooling the compressed air when it has been compressed in the first stage. Even if the compressed air is cooled between the compressions, it is often at a very high temperature after it has been compressed in the second stage. The air-cooled charge air cooler is therefore likely to have difficulty in effecting cooling of the compressed air to a temperature close to the temperature of the surroundings at which an optimum amount of air can be led to the combustion engine.